1. Field of the Invention
The invention pertains to stabilized target seeking devices and more particularly concerns active continuous-wave frequency-modulated millimeter wave radar concepts using multiple range bins for target detection and tracking.
2. Description of the Prior Art
Various prior passive and radiometric millimeter wave length ground target detection and tracking systems have demonstrated highly accurate passive target tracking. However, these passive guidance systems tend to have relatively short range and, especially at the higher microwave frequencies, and further degraded by adverse atmospheric conditions. Accordingly, recent ground target seekers have relied more heavily upon the use of both active radar and passive radiometric modes to achieve beneficial combinations of target acquisition range and tracking accuracy. Operation in the active mode desirably affords the needed target acquisition and tracking range characteristics, while the relatively short range passive operation increases final tracking accuracy, overcoming the effects of target glint and scintillation at relatively short ranges.
Operation of active tracker systems in the 30 to 300 GHz frequency range results in a more desirable combination of antenna aperture size and weather penetration capability than does operation in higher or lower regions of the spectrum. In addition, significantly wider radio frequency band width is available for clutter smoothing and precipitation backscatter decorrelation than in the lower radar carrier frequencies. Typically selected millimeter wave operational frequencies are at 35 GHz and 95 GHz, where partial atmospheric windows are found; that is, where attenuation is much lower than in other parts of the millimeter wave spectrum. Since the primary use of the seeker is to detect and to track ground targets, its antenna receives a considerable amount of energy from the terrain in addition to that it receives from the target. In the active case, it is the reflected illuminating energy. In the passive case, it is primarily terrain emitted energy. In passive operation, the more reflective and therefore less emissive metallic targets tend to reflect the cold outer space temperatures and thus appear cold as contrasted with the warmer background. In the active mode, metallic targets appear as a higher source of reflected energy than a terrain area of equal physical size. In both cases, the seeker may logically be considered to be a contrast seeker. It is highly desirable to keep the antenna beam intercept on the terrain as small as possible. Since the seeker will normally be employed in relatively small diameter airframes or vehicles, it is desirable to employ carrier frequencies as high as possible, but they must be commensurate with the required atmospheric penetration capability.
Both pulse and frequency modulated carrier wave active mode waveforms have been employed in millimeter wave contrast seekers. These have employed relatively low power, solid state transmitter devices, taking into account cost of small terminally guided vehicles and volume and power constraints. Impatt diode oscillators are employed primarily in transmitter devices employing pulse modulated waveforms. The seeker of the present invention employs a frequency modulated waveform and a Gunn diode oscillator transmitter. It has significantly unique features over the system of the U.S. Pat. No. 3,921,169 for a "Multiple Mode Radiometric System with Range Detection Capability", issued in the names of R. E. Lazarchik, R. S.Roeder, and Donald R. Runkle, Nov. 18, 1975 and assigned to Sperry Corporation. The present invention is also generally related to that of U.S. Pat. No. 4,200,871 for an "Acquisition System for Continuous-Wave Frequency Modulation Object Detector", filed in the names of R. S. Roeder and L. C. Bomar and also assigned to Sperry Corporation.
Another important function of ground target contrast seekers is signal processing to discriminate against the active mode terrain reflected clutter energy which competes with the substantially point target reflected energy. Both pulse and frequency modulation continuous wave systems can employ range resolution to reduce the active mode clutter return by effectively reducing the illuminated terrain area which would otherwise result at the antenna beam intercept. Pulse systems employ narrow pulses and time gating to achieve range resolution. These systems in general must employ wide receiver intermediate frequency band widths in order to receive the narrow pulse. Band width of at least 1/.tau. is required, where .tau. is the pulse duration in seconds. Systems employing incoherent frequency modulation wave forms can transmit energy over wide bands for effecting terrain and precipitation clutter decorrelation and still employ narrow receiver intermediate frequency band widths. This is an advantage in maximizing signal to total noise ratio (S/N.sub.t). The active mode target detection range of millimeter wave contrast seekers is usually limited by the target signal to background noise (S/N.sub.B); i.e., by the terrain reflected energy, thus indicating the desirability of minimizing antenna beam width. However, in certain environmental conditions, i.e., low terrain reflectivity and adverse atmospheric conditions, S/N.sub.r can become a range performance limiting factor. The quantity S/N.sub.r is defined as the signal to receiver noise ratio. This is particularly true if relatively long detection range is desired in a system operating at the higher millimeter wave frequencies, where higher atmospheric attenuation occurs. Thus, maximizing S/N.sub.r can be an important design factor. Range resolution in frequency modulation systems, employed for minimizing terrain and precipitation noise and for range to target/terrain measurement, is accomplished with frequency gating rather than time gating. Millimeter wave contrast seekers employing both horizontal trajectories and near vertical trajectories during target search have been demonstrated. Horizontal trajectory systems can better utilize range resolution to reduce background clutter during target search because of the antenna beam depression angle .psi. generally employed (in the order of 25 to 30 degrees from horizontal). The range resolution (range bin) intercept length on the terrain is increased over the actual range resolution by a factor of (1/cos .psi.). It will be noted that, for a completely vertical system where .psi.=90.degree., the terrain clutter reduction that can be realized from range resolution is zero. Other typical differences between millimeter wave contrast seekers for horizontal and near vertical target search mode trajectories include the search pattern employed and the related target detection signal processing implementation.
Stabilization of the seeker system is the means by which the seeker line of sight (LOS) is decoupled from and made independent of vehicle body rotation. Two methods of the prior art will be briefly described. There generally are two control axes, but only elevation is shown in the figures. In the prior art system of FIG. 1, there are two rate gyroscopes disposed on antenna 1 on mutually orthogonal axes, one for elevation motion sensing and the other for azimuth motion sensing. Any vehicle motion-induced disturbance torque, such as from bearing friction, will tend to rotate antenna 1. The rate gyroscopes immediately sense this and send an opposing signal to amplifiers and thus to the torque motors which offset the disturbance torques and hold the antenna rate very nearly to zero. There is an additional amplifier input from the millimeter wave sensor which commands antenna 1 to track the target. When the tracking error .epsilon. is zero, the rate command is zero. In FIG. 1, and more particularly with respect to the elevation control channel, any motion-induced torque tends to rotate antenna 1 about the elevation axis 9. The elevation rate gyroscope 4 senses the rotation and supplies an opposing signal to servo amplifier 3 via lead 6 and thus to the elevation torque motor 2, which thereby holds the antenna rate substantially at zero. The input from terminal 5 from the millimeter wave sensor commands the antenna to track the target. The azimuth axis error correction is similar.
In the prior art two-degree-of-freedom gyroscopically stabilized system of FIG. 2, a spinning mass 7 is used directly to stabilize antenna 1. Any disturbance torques are automatically resisted by the spin momentum of the spinning mass 7. Slight precession errors are detected by the millimeter wave sensor and cross axis torques are automatically applied. For example, if an elevation error is apparent, azimuth torquer 8 is energized through servo amplifier 3 to precess antenna 1 about its elevation error to zero. Azimuth error correction is similarly accomplished.